This invention relates to trolleys or wheel assemblies and more particularly relates to a wheel assembly capable of maintaining engagement with a rail as the wheel assembly traverses an uneven portion in the rail, which rail may be a circular rail disposed above a nuclear reactor pressure vessel for suspending service tooling from the rail to service the pressure vessel. Periodically, nuclear reactor pressure vessels require servicing, such as refueling a nuclear reactor core disposed therein. The reactor pressure Vessel includes a shell having an open end and a shell flange surrounding the open end of the shell. A hemispherical closure head sealingly caps the open end of the pressure vessel, the closure head having a closure head flange surrounding the perimeter thereof. The flange of the closure head has a plurality of spaced-apart holes therethrough. The closure head is attached to the shell by a plurality of externally threaded studs extending through the holes in the closure head flange and into the flange of the shell. A plurality of internally threaded nuts threadably engage respective ones of the studs and are tightened against the closure head flange for securing the closure head to the shell. In order to refuel the reactor core, the nuts are unthreaded from their respective studs for removing the closure head from the shell to allow access to the reactor core.
After refueling or other servicing operations, the closure head is replaced on the open of the shell such that the studs extend through their respective holes in the closure head flange. However, before the nuts are threadably run-down their respective studs, the studs are first pretension in a manner well known in the art so that the nuts will intimately engage the closure head flange when the nuts are run-down the studs. During the pretensioning process, each stud is threadably engaged with its associated nut; however, the nut is not completely run-down the stud. A stud pretensioning device is hoisted above the stud and attached to an end portion of the stud that projects upwardly from the nut. Tension is then applied to the end portion of the stud by operating the pretensioning device, so that the stud elastically lengthens a predetermined amount. After a predetermined amount of tension is applied to the stud for lengthening the stud, the nut is completely run-down the stud to intimately engage the closure head flange. The pretensioning device is then removed from the stud, such that the stud elastically returns to substantially its original length exerting a compressive force against the nut. This process is repeated for each stud and nut combination. In this manner, pretensioning each stud followed by a run-down of the nut on the stud securely attaches the closure head to the open end of the pressure vessel shell. The above process can also be used in reverse order to detach the closure head from the pressure vessel shell.
As stated hereinabove, the pretensioning device is hoisted above the stud that is to be pretensioned. In this regard, a circular rail is positioned above the closure head and a hoist mechanism is connected to the rail, the hoist mechanism engaging a cable that extends downwardly from the hoist. A grappler or hook is attached to an end of the cable for connecting the hoist mechanism to the pretensioning device. The hoist mechanism itself includes a roller assembly connected thereto for rolling the hoist mechanism around the circular rail, so that the pretensioning device can be positioned on a selected stud. Such prior art rolling assemblies may have at least one wheel engagable with the rail for rolling the hoist mechanism therealong. It is desirable that the wheel maintains traction with the rail so that the hoist mechanism travels around the rail without slippage in order to precisely align the pretensioning device with the selected stud.
However, applicants have observed that the rail may have a raised, bumpy or uneven portion therein impeding the movement of the roller assembly therealong. When such prior art roller assembly encounters the uneven portion, it may loose traction and slip. On occasion, such an uneven portion may be severe enough to halt the travel of the roller assembly, thereby necessitating time-consuming manipulation of the roller assembly to force it past the uneven portion.
Therefore, what is needed is a wheel assembly capable of maintaining engagement with a rail as the wheel assembly traverses an uneven portion in the rail, which rail may be a circular rail disposed above a nuclear reactor pressure vessel for suspending service tooling from the rail to service the pressure vessel.